Biomass may be useful as a source of renewable fuels. One type of biomass is plant biomass. Plant biomass is the most abundant source of carbohydrate in the world due to the lignocellulosic materials composing the cell walls in higher plants. Plant cell walls are divided into two sections, primary cell walls and secondary cell walls. The primary cell wall provides structure for expanding cells and is composed of three major polysaccharides (cellulose, pectin, and hemicellulose) and one group of glycoproteins. The secondary cell wall, which is produced after the cell has finished growing, also contains polysaccharides and is strengthened through polymeric lignin covalently cross-linked to hemicellulose. Hemicellulose and pectin are typically found in abundance, but cellulose is the predominant polysaccharide and the most abundant source of carbohydrates.
Most transportation vehicles require high power density provided by internal combustion and/or propulsion engines. These engines require clean burning fuels which are generally in liquid form or, to a lesser extent, compressed gases. Liquid fuels are more portable due to their high energy density and their ability to be pumped, which makes handling easier.
Currently, bio-based feedstocks such as biomass provides the only renewable alternative for liquid transportation fuel. Unfortunately, the progress in developing new technologies for producing liquid biofuels has been slow in developing, especially for liquid fuel products that fit within the current infrastructure. Although a variety of fuels can be produced from biomass resources, such as ethanol, methanol, biodiesel, Fischer-Tropsch diesel, and gaseous fuels, such as hydrogen and methane, these fuels require either new distribution technologies and/or combustion technologies appropriate for their characteristics. The production of these fuels also tends to be expensive and raise questions with respect to their net carbon savings.
Carbohydrates are the most abundant, naturally occurring biomolecules. Plant materials store carbohydrates either as sugars, starches, celluloses, lignocelluloses, hemicelluloses, and any combination thereof. In one embodiment, the carbohydrates include monosaccharides, polysaccharides or mixtures of monosaccharides and polysaccharides. As used herein, the term “monosaccharides” refers to hydroxy aldehydes or hydroxy ketones that cannot be hydrolyzed to smaller units. Examples of monosaccharides include, but are not limited to, dextrose, glucose, fructose and galactose. As used herein, the term “polysaccharides” refers to saccharides comprising two or more monosaccharide units. Examples of polysaccharides include, but are not limited to, cellulose, sucrose, maltose, cellobiose, and lactose. Carbohydrates are produced during photosynthesis, a process in which carbon dioxide is converted into organic compounds as a way to store energy. The carbohydrates are highly reactive compounds that can be easily oxidized to generate energy, carbon dioxide, and water. The presence of oxygen in the molecular structure of carbohydrates contributes to the reactivity of the compound. Water soluble carbohydrates react with hydrogen over catalyst(s) to generate polyols and sugar alcohols, either by hydrogenation, hydrogenolysis or both.
U.S. Publication No. 20080216391 to Cortright et al. describes a process for converting carbohydrates to higher hydrocarbons by passing carbohydrates through a hydrogenation reaction followed by an Aqueous Phase Reforming (“APR”) process. The hydrogenation reaction produces polyhydric alcohols that can withstand the conditions present in the APR reaction. Further processing in an APR reaction and a condensation reaction can produce a higher hydrocarbon for use as a fuel. Currently APR is limited to feedstocks including sugars or starches, which competes with the use of these materials for food resulting in a limited supply. There is a need to directly process bio-based feedstocks including “biomass”, or lignocellulosic feedstocks, into liquid fuels.